Heretofore, there have been proposed various types of recording media for recording various kinds of information signals such as video signals, music signals, computer programs or the like. Such recording media are used in the form of a so-called magnetic disk, optical disk or semiconductor memory. Also, information signals to be recorded to these recording medium are digital and analog.
For example, an optical disk having digital data recorded therein includes a disc substrate formed from a transparent material and on which there are formed very tiny holes corresponding to information signals as digital data, that is, pits, and a reflective layer formed on the pits. In this optical disk, a light beam is irradiated to the pits through the disk substrate and a reflected light from the pits is detected, to thereby read information signals recorded by the pits.
Information signals recorded on the recording medium such as the above-mentioned optical disk can easily be copied as they are, namely, in the digital form, to another recording medium. For example, use of the method and apparatus for forming pits on an original optical disk permits to form the same bits as those on the original optical disk onto another optical disk on the basis of digital data read from the original optical disk. Also, the digital data read from the optical disk can be recorded to a recording medium of any other type such as a magneto-optical disk or the like.
Different from analog data, the digital data thus copied are quite the same in quality of images and sounds demodulated from the copied digital data and in computer operation based on the digital data as the original digital data. That is, the copied digital data can be said to have the same value and quality as the original digital data.
However, if data can be copied as above without any degradation in value and quality, the copyright for the digital data will not possibly be protected adequately.
More specifically, a price of a recoding medium having digital data normally recorded therein and which is to be normally sold includes a so-called copyright royalty as a charge for use of the digital data recorded in the recording medium. Payment of the charge for use to the copyrighter for the digital data is the protection of the copyright. If the digital data copied without permission of the copyrighter is the same in value and quality as the original digital data, however, a person going to use the digital data will not use a low-price recording medium having the digital data copied thereto without buying the regular recording medium at the regular price including the royalty. Thus, the copyrighter for the digital data will not be paid appropriately for a number of persons actually having the digital data as well as for a number of times the digital data has been used.
Recently, to prevent digital data of a copyrighter from being illegally copied, there has been demanded a recording medium from which data cannot easily be copied.